ACTIVATION OF LOCUS-COERULEUS SOMATOSTATIN RECEPTORS INDUCES AN INCREASE OF GROWTH-HORMONE RELEASE IN MALE-RATS

Noradrenergic neurons of the locus coeruleus (LC) negatively regulate the endogenous rhythmicity of growth hormone (GH) secretion, These neu rons express high concentrations of receptors for somatostatin (SRIH) and galanin (GAL), two neuropeptides which can affect electrical activ ity of LC neurons and also centrally modulate plasma GH levels. We thu s investigated whether somatostatin and galanin receptors located in t he LC are involved in GH regulation, Pulsatile patterns of endogenous GH secretion were monitored after unilateral infusion of the peptides into the lateral ventricle (ICV) or into the LC, after lesion of contr alateral LC neurons by 6 hydroxydopamine. Neither unilateral LC lesion s nor administration of saline affected GH release, When administered ICV, both SRIH (5 mu g/mu l/15 min) and GAL (1 mu g/mu l/15 min) resul ted in a marked increase in GH secretion. Infusion of SRIH into the LC induced a significant but weaker stimulation of plasma GH as compared to ICV injections, In contrast, infusion of GAL into the LC was ineff ective. These results indicate that somatostatin can exhibit direct ef fects on noradrenergic neurons of the LC involved in GH regulation, wh ereas central effects of galanin on the hormone are mediated by distin ct structures.